My mother is exactly the same way. She's 5'6' so only about an inch shorter than I am, but she has never been over 120 lbs or so. Ever. I'm a more muscular frame in general so I sit in the 145-150 zone (now that I've returned to a normal weight thanks to paleo), but I remember sitting there with her at the dinner table and eating exactly the same thing she was and yet somehow, I ended up at 240 lbs. That was the day I decided something was wrong.

I stopped eating the things she does, and the weight melted away as if it never was. Now I eat easily double the calories I used to and probably triple what she is, but I look more like her now. It defies logic, or at least, conventional logic.

The only explanation I have is that with each generation, the storing of fat becomes more efficient and its helped along by processed foods that lend themselves well to the task. Human evolution is rebelling against the direction we are trying to force it in.